1. Field of the Invention
The present invention relates to a position detection method, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
A projection exposure apparatus such as a stepper and a scan type projection exposure apparatus such as that of the step & scan scheme have been known as exposure apparatuses used to manufacture, for example, a semiconductor device, a liquid crystal display device, and a thin film magnetic head by lithography (Japanese Patent Laid-Open No. 2005-302825).
An exposure apparatus will be simply explained with reference to FIG. 1 herein.
A single-wavelength oscillation laser beam such as a KrF excimer laser beam or an ArF excimer laser beam having a shorter wavelength than it as extreme ultraviolet light is used for an exposure light source LS. A light beam emitted by the exposure light source LS illuminates a predetermined region on a reticle RT serving as a pattern original via an illumination optical system IL. A pattern to be transferred is formed on the reticle RT. The pattern (for example, a microcircuit pattern) formed on the reticle RT is projected onto a wafer W by a projection optical system PO.
The exposure apparatus is required to have a resolving power close to a theoretical limit. To meet this need, the exposure apparatus includes a mechanism which measures factors (for example, the atmospheric pressure and ambient temperature) that influence the resolving power, and corrects the imaging characteristics of the projection optical system PO in accordance with the measurement result. The numerical aperture of the projection optical system PO is set large in order to attain a high resolving power, so the depth of focus is considerably small. A focus position detection system FS of the oblique-incidence detection scheme (to be referred to as a focus detection system hereinafter) measures the three-dimensional shape of the entire surface of the wafer W, and adjusts the level, which is optimum for exposure, of the projection optical system PO in the optical-axis direction (to be referred to as the z direction hereinafter).
Along with an improvement in the resolving power, higher overlay accuracy is also required. An offaxis alignment detection system OA (to be referred to as an alignment detection system hereinafter) set outside the optical axis of the projection optical system PO is used for overlay. The alignment detection system OA observes a plurality of alignment marks formed on the wafer W, and calculates and corrects the amount of misalignment in a shot region on a plane (to be referred to as the x-y plane hereinafter) perpendicular to the optical axis of the projection optical system PO.
The optical axis of the projection optical system PO which actually performs exposure and that of the alignment detection system OA have a distance called the base line amount between them. When the base line amount changes, an error occurs as the marks are moved under the projection optical system PO after measurement by the alignment detection system OA. To attain stable alignment with a higher accuracy, an optical position detection system CA of the TTL-AA (Through The Lens Auto Alignment) scheme measures and corrects the change in the base line amount. The optical position detection system CA of the TTL-AA scheme measures the relative position between the reticle RT and the wafer W using, for example, exposure light having propagated through the reticle RT and projection optical system PO. Light emitted by a light source in the optical position detection system CA illuminates an alignment mark (not shown) formed on the reticle RT. The light reflected and scattered by this mark forms an image on the image sensing surface of an image sensor in the optical position detection system CA. Detection light transmitted through a transparent region other than the alignment mark on the reticle RT reaches an alignment mark and stage reference mark on the wafer W via the projection optical system PO. The light reflected and scattered by these marks forms an image on the image sensing surface of the image sensor in the optical position detection system CA upon being transmitted through the transparent region other than the alignment mark on the reticle RT via the projection optical system PO. The alignment mark on the reticle RT and the alignment mark and stage reference mark on the wafer W can be observed simultaneously. This makes it possible to measure the relative positional relationship (in the x and y directions perpendicular to the optical axis of the projection optical system PO) between the reticle RT and the wafer W, and the conjugate relationship (focusing) between the reticle RT and the wafer W.
A reticle alignment optical system RA serves to detect whether the relative position between a reticle stage RS and the reticle RT is aligned. The reticle alignment optical system RA observes a reticle reference mark formed on the reticle stage RS and the alignment mark formed on the reticle RT in the same field, thereby measuring their relative position and aligning the reticle RT.
Optical position detection systems such as the alignment detection system OA and the optical position detection system CA of the TTL-AA scheme must adjust the focus (the position in the optical-axis direction) of the alignment mark with respect to the detection systems before measuring the position of the alignment mark. For this reason, the signal output from the image sensor, which is based on the irradiation of the mark to be detected, is monitored while driving the mark in the optical-axis direction. An optimum focus position is then detected based on evaluation values such as the driving position and the contrast of the output signal, thereby detecting the position of the alignment mark at the detected optimum focus position. For example, one known method acquires a contrast curve describing the relationship between the driving position and the contrast of the output signal, and determines the peak position of the contrast curve, at which the contrast is maximum, as the focus position.
However, depending on the illumination condition of the optical position detection system, the contrast curve often has not a single peak but a plurality of peaks. The states of individual marks vary due to manufacturing errors, for example, between shots on a wafer, between wafers, and between lots, so a peak position serving as a reference of the focus position is unstable when the contrast curve has a plurality of peaks. In this case, the detected focus position changes depending on the process state, and it also largely changes between shots and between wafers.
As shown in FIG. 9, a defocus characteristic may remain in the alignment detection system OA as an optical system assembly and adjustment error. This characteristic represents a shift in the position (in the in-plane direction on the wafer surface) of the detected mark, which depends on the focus position. Therefore, along with a large variation in the focus position in alignment, the measurement value of the alignment mark varies due to the defocus characteristic, which has a large adverse influence on the alignment accuracy.